title: Changes in muscle stiffness produced by motor units of different types in peroneus longus muscle of cat.
abstract: 1. The effects of maximal tetanic contractions of varying numbers of motor units of the same type [slow (S), fast fatigue-resistant (FR), or fast fatigable (FF)] on the mechanical responses to muscle stretch were studied in the peroneus longus muscle of anesthetized cats. 2. Two types of stiffness measurements were made: 1) an average stiffness, defined as the tension change from the beginning to end of a 0.5-mm ramp stretch; and 2) a dynamic stiffness, defined as the ratio of peak-to-peak tension to amplitude of a maintained 85-microns sinusoidal stretch at frequencies of 10-80 Hz. 3. Contractions of slow and fast units elicited different increases in average stiffness. Type S units, although developing much smaller tetanic tensions than fast ones, produced a resistance to stretch comparable with or greater than that of fast units developing much higher tensions. 4. For comparable tetanic tensions, slow units also elicited a greater dynamic stiffness than fast units. During sinusoidal stretch, changes in muscle tension led changes in muscle length during contraction of S units, but the reverse was observed for frequencies 30-50 Hz during contraction of FF units. This suggests that the latter perform oscillatory work on the driving apparatus. 5. Type S units, whose low-threshold motoneurons are the first to be recruited, appear well adapted to play a role in posture and in slow movements because of the resistance they offer to forces tending to change joint position or to oppose the progression of slow movements.
authors.affiliation: Laboratoire de Neurophysiologie, Collège de France, Paris.
authors.forename: J
authors.initials: J
authors.lastname: Petit
authors.forename: G M
authors.initials: GM
authors.lastname: Filippi
authors.forename: F
authors.initials: F
authors.lastname: Emonet-Dénand
authors.forename: C C
authors.initials: CC
authors.lastname: Hunt
authors.forename: Y
authors.initials: Y
authors.lastname: Laporte
mesh_terms: D000818:Animals; D002415:Cats; D009119:Muscle Contraction / Q000502:physiology*; D009132:Muscles / Q000033:anatomy & histology*